Method for fabricating radiation detectors



N. ANTON I 2,923,586 METHOD FOR FABRICATING RADIATION DETECTORS Feb. 2,1960 Original Filed July 15, 1953 ig-ii INVENTOR .JVcTcoZas flnfan ATTOR N EYJ' United States Patent FABRICATING RADIATION DETECTORS NicholasAnton, Brooklyn, N.Y.

5 Claims. (Cl. 316-19) METHOD FOR 7 The present invention is related tothe art including detectors of corpuscular electromagnetic radiation andis particularly concerned with methods for fabricating alpha and betaray detectors of the proportional and Geiger counter types.

This application is a division of my copending application No. 367,669for Radiation Detectors, filed July 13, 1953.

A customary form .of radiation detector particularly suitable for alphaand/or beta measurement is in the form of a metallic housing with aradiation-permeable window in one wall and containing a gas or vaporadapted to be ionized by incoming radiation passing through the window.An electrode is located within the housing and an electric field isimpressed between the electrode and the housing, the electrode servingas a collector of ions generated by the radiation. The present inventionis concerned with improvements in this type of radiation detector andmethods of manufacture therefor.

Alpha rays are swiftly moving atoms of helium stripped of their orbitalelectrons. The alpha rays emitted by radioactive decay of heavy elementshave initial velocities of three percent to six percent of the velocityof light, which is equivalent to initial kinetic energies of 2 to 8 mev.Higher energy alpha rays can be obtained by artificial acceleration.Alpha rays expend their energy by ionizing internally the matter throughwhich they travel. Their mean range in standard air is approximately 1.5to 7.5 centimeters. The range in solids or liquids is much shorter andis usually expressed in thicknesses equivalent to one centimeter of air.For convenience, these thicknesses are often givenin milligrams percentimeter. The air equivalent of aluminum is 1.2 mg. cm. of mica 1.4mg./cm. of gold 1.6 mg./cm. It can be seen, therefore, that a :mica filmof 3.5 mg./cm. will completely absorb all alpha particles of energiesless than 2.5 mev. Assuming the density of mica to be 2.8, a 3.5 mg./cm.sheet is only .0005 inch thick.

Beta rays are high speed electrons spontaneously emitted by the atoms ofradio active materials. Beta particles emitted by a radio activesubstance do not have uniform velocities, but their energies range fromzero to a maximum and comprise a spectrum which is specific to eachmaterial. One of the most useful of the common radioactive isotopes iscarbon 14 because of its extremely long half-life. The maximum energy ofcarbon 14 beta particles is .154 mev. and their average energy is .050mev.

Beta rays expend their energies by ionizing the medium through whichthey travel. Roughly speaking, their absorption depends upon the numberof atoms the beta ray is passing in the material. In a square centimeterof any material having the same weight there are about the same numberof atoms. For this reason, absorptive thicknesses are usually referredto in milligrams per centimeter without mention of the material if theelements have compositions of low atomic number. An

absorber of 1.4 mg./cm. will transmit 20% of the impinging betaparticles of .050 mev. average energy, while an absorber of. 4 mg./cm.will transmit only 1.5% not them. An absorber of 1.4 ing/cm; willtransmit 68% of impinging beta particles of .154 mev. energy, while anabsorber of 4 mg./cm. will transmit only 33% of them.

From the above, it is very apparent that one of the most importantrequirements of an alpha and/or beta counter is an extremely thinenvelope (or section of it, such as a window) which will permit theentry of the greatest possible number of charged particles into itsionizable interior atmosphere.

It is well known that radioactive decay is a statistical phenomenon. Theaccuracy of the count depends therefore upon the total number ofparticles counted. When the level of radioactivity is low, such as anaverage emission of a few particles per square centimeter per minute,

and the desired accuracy is high, it is necessary to cover.

with the counter as large a surface of the emitter as possible. Thislast requirement, i.e., a large counter, creates difiiculties because ofthe background count which is due to cosmic rays, natural gamma rays,emission fromsurrounding materials, contamination or the like. Aneflicient alpha and/ or beta counter-should possess a large andextremely thin window and a ratio of window surface to counter volume ashigh as possible, whichin turn decreases the effect of the backgroundcount. Long tedious hours of counting are necessary in biological tracerwork to separate background count from the desired count whenconventional counters are used.

Common forms of window materials used in so-called end or side windowdetectors are organic films, metallic films or thin mica splittings.Organic films can be ob: tained in thicknesses which measure only a fewmicrons. These films suffer from the disadvantage of being very weak, ofbeing usually affected bymoisture, of being unable to withstandtemperatures in excess of C., and of being dissolved by some of thedesirable quenching gases used within the envelope. Together with othernon-conductive films, they allow a charge to build up on them which cancause spurious counts to be generated in the tube.

Metal foils, while notexhibiting most of the objection? able featureslisted above, cannot be obtain-ed thin enough to be usable in themeasurement of low energy beta rays while maintaining vacuum tightness.Aluminum and aluminum alloy films thinner than .0003 inch have beenfound to be too porous for use. Although it is usually possible to finda few pieces of satisfactory .0003 inch thin disks one centimeter indiameter out of several hundred, it is impossible by present practice toobtain thinner or larger windows.

Mica, whether natural or artificial, can be split into extremely thinsheets. By careful handling, sheets '6 inches in diameterand only .0003inch thick or inch disks only 000% inch thick can be obtained. TheseWindows, although very delicate, are sufliceintly strong .to be handled.However, they cannot withstand pressure differentials required byconventional end mica window counters and necessary for sealed-offoperation, and are commonly used only in the so-called free flowcounters. Conventional mica window tubes suflerfrom the disadvantagethat in use an electrostatic charge collects on the window surface,seriously distorting the field within the counter, which makesthe-counter unreliable and sometimes inoperative. p l l Thedisadvantages of the'conventional mica window detectors fabricated bypreviously known methods are overcome by the present invention whichincludes a nurn ber of separate features advantageous.separately or incombination to improve the construction and method .of. fabrication ofradiation detectors of the present type.

According to the present inventionthe window (of, mica or other materialwhich mayhave ream/enters strength) is coated in suitable manner, onboth sides or only on the inside surface if desired, as by sputtering orevaporating, with a metal film which is made so thin as to havesubstantially no effect on the permeability of the Window to theradiation to be detected. In addition, the casing is provided with acentral internal electrically conducting post which extends generallycoaxial to the casing to provide both a support for the window at itscenter and an electrical connection which joins the film to the cathodeenvelope forming, thus, for electrostatic purposes a closed container.As a further feature, the casing is especially formed so that with itscentral post or stud it has an internal enclosed space substantially inthe shape of a toroid, and the internal electrode is formed as ,a ringor circular wire symmetrically located within the toroidal space,whereby a highly uniform electric field is provided. In some instances,where much larger detectors are desired, the central post may besupplemented by a group of concentric cylinders to provide a pluralityof supporting structures for the window, which can then be relativelythin and weak. In such case a ring-shaped electrode is located betweenthe center post and the adjoining supporting cylinder, or between eachadjoining pair of cylinders, or between the outermost cylinder and thecasing wall.

The advantages and objects of the present invention will become moreapparent from consideration of the following detailed description of thepreferred forms of the invention, taken in conjunction with the appendeddrawing, in which:

Figure 1 is a longitudinal cross-sectional perspective view of one formof radiation detector fabricated according to the invention;

Figure 2 is a top view of the device of Figure 1 with the window removedand viewed along the line 22 of Figure I;

Figure 3 is a longitudinal cross-sectional perspective view of a largerform of detector fabricated according to the present invention; and

Figure 4 is a top view of the device of Figure 3 with the window removedand viewed along line 4--4 thereof. Figures 1 and 2 illustrate a simpleform .of radiation detector fabricated according to the presentinvention.

This detector comprises an electrically conductive hous-' ing or casing11 open at one end with a window 12 thereacross and having a ringelectrode 13 insulatedly supported within the casing 11. The casing 11is formed generally of cylindrical shape with an outer wall 14 and acenter post or supporting stud 16 extending axially within outer wall14, The outer wall 14 and the stud 16 are formed so that the space 17enclosed by the cas ing 11 and stud 16 is of substantially toroidalform. For example, the casing 11 may be formed of a solid cylindricalblock of I metal having the toroidal space 17 machined out therefrom.The casing 11 is preferably of metallic material, such as No. 446stainless steel.

The window 12 is formed of a thin radiation-permeable material which canbe coated with a metallic film. It is preferably of mica but may beformed of nylon, cellophane or other synthetic plastic of preferably lowmolecular weight. In the following, it will be assumed for illustrativepurposes that the window is of mica. A thin metallic coat 18 is appliedto the inner face of the window 12 before its assembly with the casing11. This metallic coat is of any suitable material which will notchemically combine with the gas or vapor which may be used within thecasing, such as alcohol, ether, chlorine, bromine or other substance.For example the coating may be of chrome-iron, chromium, platinum orother suitable metal. It may be applied by sputtering or evaporating orby painting in emulsion form. The window 12 is made as thin as ismechanically possible with the type of construction and dimension used.

- A ring or circular-shaped electrode 13 is symmetrically located withinthe casing 11 .so that in cross-section the electrode 13 is centrallylocated with respect to the generating circle of the toroidal space 17.In this way maxi-..

mum symmetry of electric field is obtained. The electrode 13 issupported from the casing 11 in any suitable desired manner. Thus, asillustrated, the electrode 13 is provided with a plurality of equallyspaced supporting rods 23 extending radially outwardly therefrom. Eachof these rods 23 is mounted in the outer. casing wall 14 in aninsulating bushing 24 formed of Alsimag or other similar insulatingscalable material. One or more of the supporting rods 23 extendscompletely outwardly of the casing 11 to form a terminal 25. bushings 24and supports 23 are shown, it will be understood that any desired numbermay be utilized. In particular, for a small detector, a single supportmay prove to be adequate.

It will be seen that the casing 11 is also provided with a recessedledge or shoulder 19 extending just inside the open face of the casing.11, in which the window 12 rests. In assembly, the edge of the windowis painted with a suspension of glass powder in water. The glass isselected to have the same coefiicientof expansion of mica and as themetal of the casing. For example, the

glass may be Corning 7570 or Kimbal R6.' The electrode 13 is also thenplaced in position with the bushings 24 in place. The assembly is thenfired at approximately 650 centigrade, which fuses the powdered glassand joins it to both the mica window and to the metal casing to providea seal shown at 21 in Figure 1. As shown in this figure the ledge 19 isslightly depressed relative to the top of the post 16, so that uponsealing and upon evacuation of gas or vapor filling the post 16 is incontact with the.

coating 18 on window 12. At the same time the Alsimag bushings 24 arealso sealed to the supports 23 and to the casing 11. It will be seenthat the post 16 conductively connects the film 18 to the casing 11,even though the seal 21 may offer insulation.

An exhaust tubulation 22 is provided in the casing 11 to permit the airtherewithin to be exhausted and any desired vapor or gas to be enclosedwithin the'casing, whereupon the tubulation 22 is sealed off to retainthe desired condition of gas or vapor pressure in the casing.

Figures 3 and 4 show a modified form of radiation detector fabricatedaccording to the present invention suitable for larger size detectors.The construction here is essentially similar to that of Figures 1 and 2,except that for a larger size window 12 a further support andcharge-dissipating path is provided in the formof a generallycylindrical window support 16a located between the central stud 16 andthe casing wall 14. Also the support 16a as well as thecentral stud 16and the outer wall 14 are formed or fabricated to provide a plurality oftoroidal spaces 17, 17a, and each such toroidal space contains its ownring electrode 13, 13a symmetrically located therewithin. supported fromits adjoining cylindrical stud 16a by the supporting rods 23 as inFigure 1 while the outermost electrode 13a is similarly supported bysupports 23a. One or more of the supports 23 may extend to ringelectrode 13a. It will be understood that the supports 23, 23a may besuitably spot welded or soldered to the rings 13, 13a after they arelocated within the spaces 17, 17a. Insulating bushings 24, 24a areutilized in a fashion similar to Figures 1 and 2. It will be understoodthat where supports 23 are fixed to electrode 13a and sufiicientrigidity of support exists, it is unnecessary to utilize the innermostbushings 24. The construction and method of fab-- While threeinsulating.

The innermost ring electrode 13 is;

While the present invention has been illustrated as applicable toradiation detectors utilizing toroidal spaces 17, 17a, in somecircumstances it may be advantageous to simplify the construction ofthese detectors, in which case the post 16 may be a solid rightcylinder. The inner wall of the casing 14 may be also a right circularcylinder and any intermediate supporting cylinder such as 16a may alsobe a thin-walled right circular cylinder. Also, other desired forms thancylindrical may be used for the detector. Furthermore, while it ispreferred to fabricate the casing and window supports integrally from asingle metallic block, it will be understood that the casing andsupports may be formed in any desired and convenient number of separateparts which can be suitably secured together as by soldering, welding,brazing, fusing, or the like, to provide a complete casing with allparts electrically interconnected.

While the above invention has been described particularly with respectto insulated windows made conductive by a metallic coating, it may alsoutilize thin metallic film windows, in which the center posts such as 16and supports such as 16a permit use of much thinner films withoutreducing mechanical strength.

It is further to be understood that the above description is simplyillustrative of the present invention. Since many other apparentlywidely differing constructions and various methods of construction maybe readily conceived without departing from the spirit of the presentinvention, the above description is not to be construed in a limitingsense and the scope of the invention is defined solely by the appendedclaims.

What is claimed is: t

l. The method of sealing a mica window coated with a conductive materialin vapor-tight manner to a stainless steel casing, having a centralconductive support therein with said support in conductive relation tosaid conductive material coating comprising the steps of applying aliquid suspension of glass to said window, assembling said window tosaid casing, firing said assembly to fuse said glass and join saidwindow to said casing, and reducing the pressure within said casing toforce the conductive coating of said window against said support to forman intimate electrical contact therewith.

2. The method of manufacturing a radiation detector having a mica windowcoated with a conductive material, 4

a stainless steel casing, and a central conductive support therein withsaid support in conductive relation to said gonductive material coatingcomprising the steps of ap:

plying a thin conductive metal coating on said mica window, applying aliquid suspension of glass to said window, assembling said window tosaid casing, firing said assembly to fuse said glass and join saidwindow to said casing, producing a reduced pressure atmosphere withinsaid casing to force the conductive coating of said window against saidsupport to form an intimate electrical contact therewith and sealing offthe interior of said casing.

3. The method of manufacturing a radiation detector having a mica windowcoated with a thin layer of conductive metal and a metal casing having acentral conductive window support therein with said support inconductive relation to said layer ofconductive metal comprising thesteps of applying a liquid suspension of glass to said window,assembling said window to said casing, firing said assembly to fuse saidglass and join said window to said casing in a vapor tight manner, andreducing the pressure within said casing to force the conductive coatingof said window against said support to form an intimate electricalcontact therewith.

4. The method of manufacturing a radiation detector having a mica windowcoated with a conductive material, a conductive casing, and a centralconductive support therein with said support in conductive relation tosaid conductive material coating comprising the steps of applying aliquid suspension of glass to said window, assembling said window tosaid casing, firing said assembly to fuse said glass and join saidwindow to said casing, and reducing the pressure within said casing toforce the conductive coating of said window against said support to forman intimate electrical contact therewith.

5. The method of manufacturing a radiation detector having a flexiblewindow with a conductive surface, a casing and a central conductivewindow support therein with said support in conductive relation to saidconductive surface comprising the steps of applying a non-conductivecementing medium to said window, assembling said window to said casing,and reducing the pressure within said casing to force the conductivesurface of said window against said support to form an intimateelectrical contact therewith.

References Cited in the file of this patent UNITED STATES PATENTS2,629,093 Pask et a1. Feb. 17, 1953 FOREIGN PATENTS l,053,46Q FranceSept. 30, 19 53

